Understanding Synaptic Transmission

Synaptic Transmission

In order to build muscle it is important to know how the cells of the body work. The cellular machinery is complex. You might have read my posts in the past regarding how a muscle contracts. In those posts I talk about synapse. Now I would like to go more in depth in discussing the synaptic transmission.

As discussed previously the basis of a muscle contraction occurs when there is communication between neurons and muscle fibers. This communication junction is called synapses. There are two types of synapse electrical and chemical. The chemical synapse being the most prevalent.

Acetylcholine is one of the neurotransmitters responsible for initiating an excitatory response within the neuron. When a nerve impulse fires off it travels down to the presynaptic axon terminal, this is the beginning of synaptic transmission. This electrical surge initiates the depolarization of the presynaptic membrane, which in turn releases neurotransmitter release and opens up the receptors on the postsynaptic membrane.

Synaptic vesicles float in the cytoskeleton of the cytoplasm. Free vesicles travel to the plasma membrane to help assist in the priming reaction of fusion, among the vesicular and plasma membranes. The membranes of the synaptic vesicles pull together with the help of protein complexes, known as Snares. These snares are dispersed on the vesicle and presynaptic membrane.

When the axon terminal depolarizes, voltage-gated calcium channels open, and calcium ions rush into the axon terminal. Through this process synaptotagmin is also produced. Synaptotagmin is when calcium ions bind to a free floating protein on the synaptic vesicle membrane.

Calcium binding to synatotagmin at the synaptic vesicle closest to the active zone, are drawn closer to the presynaptic membrane. Once this occurs the vesicles then fuse with the axon terminal membrane, and release their neurotransmitter into the synaptic cleft. The transmitter molecules bind to distinct receptor molecules in the postsynaptic membrane.

How the postsynaptic cell responds to excitation or inhibition depends upon the type of neurotransmitter and receptor combination. Think of this like a lock and key sequence. Different keys open up different doors. Since I am talking about acetylcholine, ACH is permeable to sodium. When ACH binds, the channel opens and sodium ions enter the postsynaptic cell. This sequence produces a excitatory postsynaptic response.

The excitatory postsynaptic response is quick. Transmitters are inactivated or dispersed from the synaptic cleft quickly to accurately follow the presynaptic input signal. When ACH releases into the postsynaptic cleft it leaves an enzyme (acetylcholinesterase), which breaks down ACH into two parts; choline and acetate. This release ACHe from the receptors causes the channels to close. Setting up for the next transmission response. Not all neurotransmitters are broken down by enzymes in the synaptic cleft. Many transmitters are consumed by protein transporters. This is known as reuptake. Reuptake helps recycle the transmitter molecules keeping the neuron repolarized allowing for the next synaptic transmission.

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Daryl Conant, M.Ed